The poorest weldability among the given options is high carbon steel.

High carbon steel is known for its poor weldability compared to low carbon and medium carbon steel. This is due to several factors that affect the welding process and the resulting weld quality.

1. Carbon content:
High carbon steel contains a higher percentage of carbon compared to low and medium carbon steel. The increased carbon content makes the steel harder and more brittle, which is detrimental to the weldability. The high carbon content leads to the formation of brittle carbides during welding, resulting in a higher susceptibility to cracking and decreased ductility.

2. Heat input:
During the welding process, heat is applied to fuse the base metals together. High carbon steel has a higher thermal conductivity compared to low and medium carbon steel. This means that more heat is required to achieve the desired weld joint. The high thermal conductivity of high carbon steel can result in increased heat input, leading to a larger heat-affected zone (HAZ) and potential distortion or cracking.

3. Hardness:
High carbon steel has a higher hardness compared to low and medium carbon steel. The high hardness makes high carbon steel more prone to cracking during the welding process. The welding process introduces localized heating and cooling cycles, which can result in the formation of brittle microstructures in the weld zone.

4. Preheating and post-weld heat treatment:
High carbon steel often requires preheating before welding to reduce the risk of cracking. The preheating helps to slow down the cooling rate and minimize the formation of brittle microstructures. Additionally, post-weld heat treatment may be necessary to relieve residual stresses and improve the weld's mechanical properties. These additional steps increase the complexity and cost of welding high carbon steel.

Conclusion:
In summary, high carbon steel has the poorest weldability among the given options due to its higher carbon content, increased hardness, higher thermal conductivity, and the need for additional steps such as preheating and post-weld heat treatment to mitigate the risk of cracking. Welding high carbon steel requires careful control of welding parameters and the use of specialized techniques to ensure a successful weld joint.